Two Dimensional Field Reversed Equilibria with Plasma outside the Separatrix

Abstract

Two dimensional equilibria of field reversed configuration with plasma outside the separatrix is investigated. By numerical calculation, it is found that the separatrix (ψ=0) becomes spindly as β_s (beta ratio on the surface ψ=0) increases. Remarkable is that the surface ψ=0 becomes an open surface and no more the separatrix as β_s increases beyond a critical value, and furthermore, that only axially uniform configurations are obtained when β_s is greater than a second critical value. Existence of the critical values of β_s can be explained also with use of a long thin plasma model and a condition of axial force balance which takes into account the plasma outside the separatrix. The critical values obtained with this model well agree with the two dimensional numerical results as well as radii of the plasma both at the center plane and at the ends of vessel.